1. Field of Invention
This invention relates to polymer compositions containing thermoplastic starch and to a method of making these compositions.
2. The Prior Art
The blending of starch with synthetic polymers such as polyethylene and polypropylene has been the subject of increasing interest over recent years. The motivation is keen since starch is an abundant and inexpensive filler material. Moreover, starch may also impart partial biodegradability to the resulting blend.
Natural starch found in plant products can be isolated as a granular powder. It is known that natural starch can be treated at elevated temperature and pressure with addition of defined amounts of water to form a melt. Such a melt is referred to as gelatinized or destructurized starch. It is also known to mix destructurized starch with additives such as plasticizers to obtain a thermoplastic starch or TPS. It is known to mix these forms of starch with synthetic polymers and co-polymers. For example, U.S. Pat. No. 5,095,054, and Ind. Eng. Chem. Prod. Res. Dev. vol 19, p. 592 (1980) describe such a process.
Difficulties have arisen in that the presence of starch has had a negative impact on the physical properties of the resulting mixture when compared to the pure synthetic polymers. Furthermore, when starch is mixed with synthetic polymers or co-polymers, the starch domains are enveloped by the non-biodegradable synthetic polymers and consequently their biodegradability is significantly reduced.
A biodegradable material can be defined as one that is able to be converted to CO2 and H2O by certain common microorganisms. It is further unknown in the art to achieve mixtures of starch with non-biodegradable polymers where the starch domains are readily accessible for environmental degradation while still maintaining good mechanical properties.
With respect to the method of preparing polymer and TPS blends, some blending studies have been reported using internal mixers. Examples of such studies are found in international application WO 90/14388, European Patents 0 554 939 and 0 327 505.
It is also known from the article entitled “Processing and characterization of thermoplastic starch/polyethylene blends”, published in Polymer, 38 (3), 647 (1997), to blend TPS and low density polyethylene (LDPE) in a continuous process using a co-rotating arrangement of a twin-screw extruder fed on one side by a single-screw extruder. The side extruder is used to prepare the TPS. The main extruder is used to prepare the LDPE melt which is combined with the TPS melt. However, such process results in TPS/LDPE blends having poor physical properties including the presence of water and of bubbles. Moreover, tensile properties of the extrudate dropped off dramatically at about 10% or more of TPS content. Tests revealed that the TPS, present as a dispersed phase in the extrudate, exhibited spherical or ellipsoidal shapes. Consequently, the extrudate was not easily biodegradable since the great majority of spherical or ellipsoidal shapes were enveloped in polyethylene which is not biodegradable. In other words, the dispersed TPS phase was not continuous.
The prior art is also silent on controlling process parameters to achieve controlled morphologies of the resulting blend.
Thus, it is an object of the present invention to provide a novel method for obtaining TPS/polymer blends having controllable and improved physical properties over the prior art blends.
It is a further object of the present invention to provide an improved product comprising a blend of TPS and polymer(s) having improved physical properties over prior art blends.
It is a related object of the invention to provide an improved product wherein the TPS phase is continuous so as to allow biodegradation processes to take place within the product.
In preferred embodiments, the product is extruded sheet or blown film.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given byway of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.